In recent years, hybrid vehicles and electric vehicles are receiving attention as ecologically friendly vehicles. A hybrid vehicle uses, besides a conventional engine, a DC power supply, an inverter and a motor driven by the inverter as a mechanical power source. That is, the hybrid vehicle obtains mechanical power by driving the engine and also by converting a DC voltage from the DC power supply to an AC voltage with the inverter and rotating the motor with the AC voltage converted.
An electric vehicle uses a DC power supply, an inverter and a motor driven by the inverter as a mechanical power source.
Thus, each of the hybrid vehicle and the electric vehicle is equipped with a motor drive device including a DC power supply and an inverter. A capacitor is provided on an input side of the inverter to supply a noiseless DC voltage to the inverter. In addition, a system relay is provided between the DC power supply and the inverter (see, for example, Japanese Patent Laying-Open No. 2000-134707 and Japanese Patent Laying-Open No. 2004-303691).
In detail, Japanese Patent Laying-Open No, 2000-134707 discloses a system relay formed with a resistance R and a system relay SMR1 connected in series to a cathode of a DC power supply, a system relay SMR2 connected to the cathode of the DC power supply in parallel with resistance R and system relay SMR1, and a system relay SMR3 connected to an anode of the DC power supply. Welding of each of system relays SMR1-SMR3 is determined based on a voltage between both ends of a capacitor when system relays SMR1, SMR3 are turned on/off independently.
According to Japanese Patent Laying-Open No. 2000-134707, welding of system relay SMR3 on an anode side is first determined based on a voltage between both ends of the capacitor when only system relay SMR1, which is connected in series with resistance R, is turned on. Then, welding of system relay SMR1 or SMR2 on a cathode side is determined based on a voltage of the capacitor when only system relay SMR3 on the anode side is turned on.
Therefore, when system relay SMR1 connected in series with resistance R is welded and when only system relay SMR3 is turned on after welding of system relay SMR3 on the anode side is determined, a large-capacity battery is connected to a load side via resistance R and system relay SMR1 as well as system relay SMR3. As a result, a current flows from the large-capacity battery to the load side via resistance R and system relay SMR1 as well as system relay SMR3, and charges are supplied to the capacitor.
In this situation, since charges are supplied from the battery to the capacitor so as to compensate for discharging to a load, the voltage between both ends of the capacitor is not decreased and kept to substantially a constant level. Therefore, a large amount of current for supplying charges to the capacitor continuously flows through resistance R which is originally formed with specifications considering a resistance value and heat resistance sufficient to prevent an instantaneous large current (inrush current), and thus resistance R may be damaged due to unexpected heat production.
An object of the present invention is to provide a power supply control device capable of determination of welding of a relay while protecting a resistance.